1. Related Technical Fields
Related technical fields include navigation devices, methods, and programs that perform map matching for navigation.
2. Related Art
On-vehicle navigation devices that find a travel route to a destination and guide the vehicle are widely used. This type of navigation device identifies the current position of the vehicle, displays the vehicle's current position on a map, and provides travel guidance. Types of sensors used for identifying the current position include gyro, vehicle speed pulse, and acceleration sensors, and an estimated path is generated from the sensor information using dead reckoning navigation. Shape matching of the estimated path and road information stored in a database is performed, and a position where the two coincide is identified as the current position. A current position mark that represents the host vehicle position is displayed at the identified current position. The matching method may assign a cost value that represents a match probability based on various information in order to each match candidate. After comparing cost values, a match is then made to the match candidate with the smallest cost value.
GPS is used for measuring an absolute position (latitude, longitude) of the vehicle. GPS information is used in sensor learning when the GPS accuracy is high, but not always used for position correction during normal updating of the current position. This is because multipath or similar effects, a weakness of GPS, may reduce the accuracy.
If the current position is identified by dead reckoning navigation in this manner, false matching of the current position may be more likely to occur for the following reasons when an angle formed by a plurality of roads branching at a narrow angle (narrow angle branching) is equal to or less than a predetermined angle, for example. Because the road following the route as judged from the road type, link shape and the like is given priority during matching, a false match may result when the host vehicle advances on a road separate from the road following the route. A false match to the wrong road may also occur depending on a heading error of the estimated path. The actual road, which has width, is expressed as a line (link) in the database. Consequently, the actual travel route and the link shape may not coincide, and matching to the correct road may not be possible even after comparing the estimated path and the link shape.
Once a false match is made at a narrow angle branching, recovery (matching to the correct road) may be delayed for the following reasons. Namely, in a case where both roads are practically parallel after a narrow angle branching, a comparison between the estimated path and the link shape shows no difference. Therefore, recovering matching to the correct road is difficult. The displayed estimated path may also be corrected so as to overlap the falsely matched link, making recovery to the correct road in such case even more difficult.
Japanese Patent Application Publication No. JP-A-2006-284499 proposes art that uses a GPS positioning point in order to avoid such false matching after a narrow angle branching. According to JP-A-2006-284499, an error circle is set centered on a GPS positioning point and has an error radius that corresponds to the DOP (dilution of precision). A host vehicle position detected within the range of the error circle is considered correct. After passing a branch point, the error circle is reduced in a stepped manner from the branch point so that candidate points in the case of a false match are quickly removed from the error circle and a recovery to the correct road can be made accordingly.